In order to wake-up a conventional passive RFID transponder chip, the rectified voltage from an RF signal picked-up by the antenna of the chip has to be greater than a power-on reset voltage previously set. When the rectified voltage across capacitors is sufficient from an RF signal picked-up by the antenna, the chip circuit is powered to communicate with a reader. Usually some power-on reset trim values can be stored in a non-volatile memory of the transponder chip. Said trim values can be applied from the non-volatile memory to a power-on reset circuit of the chip in order to trim the level of the power-on reset voltage, when the transponder chip is in operation. However, the non-volatile memory stays disabled as do several electronic parts of the chip, and cannot be read, if the rectified voltage is under the level of an untrimmed power-on reset voltage. With an initial untrimmed power-on reset voltage, which is greater than a trimmed power-on reset voltage, the distance between a reader and the RFID transponder chip must be reduced or the reader RF power increased in order to power the chip for normal operation. So the chip sensitivity of a conventional passive RFID transponder chip is poor, which is a drawback.
It is also to be noted that according to manufacturing and temperature variation of the chip, the power-on reset voltage varies resulting in a specified sensitivity. This can represent the expected worst case of the power-on reset voltage plus some margin. For an improved sensitivity specification, the power-on reset circuit has to be trimmed to reduce the variation of the power-on reset voltage. Usually the trim values are determined during the testing of the RFID transponder chips prior to shipment and stored in a non-volatile memory.
In the EP patent application No. 1 102 158 A1, it is described a device and a method for controlling the operation of an electronic system in a “grey zone”. Said operation begins after the power supply exceeds the power-on reset voltage by some amount. Said electronic system includes in particular a microprocessor able to operate at a guaranteed minimum voltage, which can be for example greater than the power-on reset voltage. In such a device, if the trim values are to be applied for the power-on reset circuit, the non-volatile memory is read after wake-up of the chip, and the trim values are applied resulting in a change from the untrimmed power-on reset voltage level to a newly trimmed power-on reset voltage level. If the untrimmed power-on reset voltage level is higher than the trimmed power-on reset voltage level, then the sensitivity is initially worse, but said sensitivity is then better after applying the trim values. This is because the power supply can be maintained at a lower level without falling below the trimmed power-on reset voltage level. In practice, it means that for the first communication the transponder chip must be brought close to the reader to awaken before it can be moved further away while staying awake, which is a drawback.
The U.S. Pat. No. 6,922,134 B1 describes a programmable trimmer for a transponder chip. Said transponder chip includes a dedicated EEPROM memory, which holds trim data values that are an integral portion of the EEPROM main memory. These trim dedicated cells of the EEPROM memory share bit lines with other non-trim cells, and are written via the shared bit lines. Trim and non-trim cells must be written at different time periods. The write circuitry must be designed for the load of both trim and non-trim cells. The read circuitry contains the sense function and is separately supplied with power. So in this case, it is necessary to wait for wake-up of the transponder chip before eventually tuning the power-on reset circuit with trim data values stored in said EEPROM memory, which is a drawback.
U.S. Pat. No. 6,980,084 B1 describes a power-on reset for a transponder chip. The power-on reset circuit of the transponder chip is used in a tire pressure monitoring system. Low power components enable the power-on reset circuit to be functional beginning at a power supply level below a sustain voltage. Other transponder chips become functional at a higher power supply level just under a start voltage. A trim bit value from a non-volatile memory controls an analog function upon which sustain voltage level is dependent. The value controls the selection of one of two sustain voltage levels, a higher level for passive mode and the lower level for active mode. The trim bit value also controls the power supply rising and falling reset trip behavior. When the reset output is high, the transponder chip is disabled and will not communicate.
In passive mode, the reset output goes low when the power supply rises above the start voltage and goes high when the power supply falls below the sustain voltage. This allows an on-chip backscatter modulator to operate starting at a higher power supply without loading and dragging down the power supply enough to cause a reset to assert high. In active mode, the reset output is low whenever the condition of the power supply greater than the sustain voltage is met. Since the transponder chip is powered by a battery, a lower reset condition is possible because the battery can supply more current into the load presented by an “on” modulator without momentarily dropping the power supply. This reset functionality is further combined with a minimum delay for asserting the reset output such that if power supply to the transponder chip increases quickly, there is enough time for analog circuits to stabilize prior to the start of communication. The concept in U.S. Pat. No. 6,980,084 does not trim the power-on reset release voltage for a rising power supply level, rather it trims the falling power-on reset assert voltage by mode (passive or active) and not even by chip, which is a drawback.